1. Field of the Invention
The present invention relates to a light emitting device using an organic light emitting element having an anode, a cathode, and a film containing an organic compound in which light emission can be obtained by applying an electric field (hereafter referred to as an “organic compound films”). In particular, the present invention relates to a light emitting device using organic light emitting elements having a driver voltage that is lower than a conventional driver voltage and having a long life. Note that the term light emitting device within this specification indicates image display devices or light emitting devices using organic light emitting elements as tight emitting elements. Further, modules in which a connector, for example, an anisotropic conductive film (flexible printed circuit, FPC), a TAB (tape automated bonding) tape, or a TCP (tape carrier package) is attached to organic light emitting elements, modules in which a printed-wiring board is provided on the tip of the TAB tape or the TCP, and modules in which an IC (integrated circuit) is directly mounted to the organic light emitting elements by a COG (chip on glass) method are all included in the category of the light emitting devices.
2. Description of the Related Art
Organic light emitting elements are elements which emit light by the application of an electric field. The light emitting mechanism is one in which electrons injected from a cathode recombine within an organic compound film with holes injected from an anode, forming excitation state molecules (hereafter referred to as “molecular excitons”), by the application of a voltage between the electrodes sandwiching the organic compound film. Energy is released when the molecular excitons return to a base sate, thereby emitting light.
Note that it is possible for the molecular excitons formed by the organic compound to be in a singlet excitation state or in a triplet excitation state, and both cases in which either excitation state may contribute to the emission of light are included within this specification.
The organic compound film is normally formed by a thin film having a thickness less than 1 μm for these types of organic light emitting elements. Further, organic light emitting elements are self light emitting elements in which light is emitted by the organic compound films themselves, and therefore a back light like that is used in a conventional liquid crystal display is not necessary. Consequently, the ability to manufacture the organic light emitting elements that are extremely thin and light is a big advantage.
Furthermore, the amount of time from the injection of a carrier until recombination in an organic compound film having a thickness on the order of 100 to 200 nm, for example, is on the order of several tens of nanoseconds when considering carrier mobility of the organic compound film, and even in a case of including a process from when the carrier recombines until light is emitted, the light emission can be reached within one microsecond. Therefore, it is one of the characteristics that light emitting elements have an extremely fast response speed.
In addition, drive using a direct current voltage is possible because the organic light emitting elements are light emitting elements of a carrier injecting type, and therefore it is difficult for noise to develop. With regard to a driver voltage, it has been reported (reference 1: Tang, C. W., and VanSlyke, S. A., “Organic Electroluminescent Diodes”, Applied Physics Letters, Vol. 51, No. 12, pp. 913-915 (1987)) that a sufficient brightness of 100 cd/m2 at 5.5 V was achieved by first taking an extremely thin film of an organic compound with a uniform film thickness on the order of 100 nm, selecting an electrode material so as to make a carrier injection barrier of the organic compound film smaller, and in addition, introducing a heterostructure (two layer structure).
Organic light emitting elements are therefore under the spotlight as display elements for next generation flat panel display elements due to their thin size light weight, high speed response, and dc low-voltage drive. Furthermore, the organic light emitting elements are of a self light emitting type and have a wide field of view, and therefore their visibility is comparatively good and they are considered to be effective as elements used in the display screens of portable devices.
An Mg:Ag alloy having a low work coefficient and which is relatively stable is used in the cathode as a method of making the carrier injection barrier with respect to the organic compound film smaller, thereby increasing the electron injection properties, in the organic light emitting elements shown in reference 1. It is thus possible to inject a large amount of carrier into the organic compound film.
In addition, applying a single heterostructure in which a hole transporting layer formed of an aromatic diamine compound and an electron transporting and light emitting layer formed of tris-(8-quinolinolate)-aluminum (hereafter referred to as “Alq3”) are laminated as the organic compound film remarkably increases the efficiency of the recombination property of the carrier. This will be explained as follows.
For example, if the organic light emitting elements have only a single layer of Alq3, then almost all electrons injected from the cathode will reach the anode without recombining with holes because Alq3 has electron transporting properties, and the efficiency of light emission is extremely bad. That is, in order to make the efficiency of single layer organic light emitting elements better (or in order to perform drive at a low voltage), it is necessary to use a material capable of transporting both electrons and holes with a good balance (hereafter referred to as a “bipolar material”). Alq3 does not satisfy this condition.
However, provided that a single heterostructure like that of reference 1 is applied, electrons injected from the cathode are blocked by an interface between a hole transporting layer and an electron transporting and light emitting layer, and are confined within the electron transporting and light emitting layer. Carrier recombination therefore occurs with good efficiency in the electron transporting and light emitting layer, and light emission having good efficiency is achieved.
Expanding upon the concept of a blocking function of this type of carrier, it becomes easy to control the carrier recombination region. For example, it has been reported that a hole transporting layer has been successfully made to emit light by confining holes within the hole transporting layer through inserting a layer capable of blocking holes (hole blocking layer) between a hole transporting layer and an electron transporting layer, (reference 2: Kijima, Y., Asai, N., and Tamura, S., “A Blue Organic Light Emitting Diode”, Japanese Journal of Applied Physics, Vol. 38, pp. 5274-5277 (1999)).
Further, the organic light emitting elements of reference 1 perform separation of functions in that hole transportation is performed by the hole transporting layer, and electron transportation and light emission are performed by the electron transporting and light emitting layer. The concept of the function separation is further expanded upon to the concept of a double heterostructure (three-layered structure) in which a light emitting layer is sandwiched by a hole transporting layer and an electron transporting layer (reference 3: Adachi, C., Tokoto, S., Tsutsui, T., and Saito, S., “Electroluminescence in Organic Films with a Three-layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L269-L271 (1988)).
An advantage of the so-called separation of functions is that the need to make one type of organic material possess several types of functions (such as the ability to emit light, carrier transporting properties, the ability to inject carriers from electrodes), and therefore separation of functions provides a wide ranging amount of freedom in molecular design and the like (for example, there is no longer a need to search unreasonably for a bipolar material). In other words, high efficiency light emission can be easily achieved by combining materials such as a material having good light emitting properties and a material with superior carrier transporting properties, respectively.
The lamination structure discussed in reference 1 therefore enjoys widespread use at present due to these advantages (carrier blocking function and separation of functions).
However, interface boundaries (hereafter referred to as “organic interfaces”) develop between each layer with a lamination structure like that discussed above because the structure is made of junction of different types of substances. Two problems which have their origin in the formation of organic interfaces are presented below.
First, one problem is hindrance of an additional reduction in a driver voltage. It has been reported for organic light emitting elements that in practice, single layer structure elements using a conjugate polymer are superior with regard to the driver voltage, and that top data power efficiency (units of 1 m/W) is maintained (compared to light emission from a singlet excitation state), (reference 4: Tsutsui, T., J. Applied Physics Society Organic Molecules—Bio-electronics Section, Vol. 11, No. 1, p. 8 (2000). (However, this is compared to light emission from a singlet excitation state, and excludes light emitted from a triplet excitation state.)
Note that the conjugate polymers discussed in reference 4 are bipolar materials, and that a level of carrier recombination efficiency equivalent to that of a lamination structure can be achieved. In practice, a single layer structure having few organic interfaces therefore shows a lower driver voltage provided that the carrier recombination efficiency can be made equivalent, without the use of a lamination structure, by a method such as using a bipolar polymer. This eventually suggests that carrier mobility in the organic interface is hindered.
In addition, another problem originates in an organic interface is exertion of influence on the element lifetime (element deterioration) for organic light emitting elements. Namely, brightness drops because the carrier mobility is impeded and charge accumulates.
No definite theory has been established regarding the mechanism of this degradation, but it has been reported that the drop in brightness can be suppressed by inserting a hole injecting layer between the anode and the hole transporting layer, and in addition, by performing ac drive at a short wavelength instead of dc drive (reference 5: VanSlyke, S. A., Chen, C. H., and Tang, C. W., “Organic Electroluminescent Devices with Improved Stability”, Applied Physics Letters, Vol. 69, No. 15, pp. 2160-2162 (1966)). This can be said to be experimental evidence that the reduction in brightness can be suppressed in accordance with eliminating charge accumulation by inserting the hole injecting layer and by using ac drive.
From the above discussion, the lamination structure has the merits of being able to easily increase the carrier recombination efficiency (carrier blocking function), and being able to increase the breadth of selection of materials (separation of functions). However, carrier mobility is suppressed due to the formation of organic interfaces, in particular interfaces which block carriers, and this in turn influences the reductions in the driver voltage and in brightness.